Based on the movie and the novel, we know that Vito Corleone thought that "Santino was a bad don rest in peace." He lacked subtlety and lost his temper which led to his downfall. Also, during the Sollozzo meeting, Sonny made an almost fatal mistake which gave Sollozzo an opening by showing interest in the drug deal. In the book, they mentioned that Santino did win some major victories in a gang war in his youth.

Would Vincent have made a better don than his father? Obviously, he learned a lot from Michael but would it be enough? Supposedly, in the rumored script for the Godfather Part IV, he would lead the family back to the world of crime and lose his life at the end.

I think Vincent would be slightly better than Sonny as he really listened to Michael whereas Sonny didn't learn as much from Vito and his temper was a major weakness.

Their leadership tenure was during different eras and circumstances. It's non-comparable.

That's true. And, Sonny spent his lifetime apprenticing directly under Vito in his gambling and labor rackets. We don't know what Vincent did while he was working for Zasa.

I think Vincent was less hot-headed than Sonny--a plus. On the other hand, Vincent didn't seem to be very smart.

Yes different times. I would go with Vincent as being a better boss, more in the financial department where Sonny would be a better boss in the war department imo. There is not that much on Vincent but he seemed to smarted up a lot more when he was with Michael.

Why are they incomparable? Every boss ruled during a different era and they all had various circumstances to deal with. Invalid statement right there in my opinion.

It's a hard pick,but I'll go with Vincent. I think he would have taken Mike's mentoring well,and even though he could be a hothead, I believe he might have been more open to advancing the Corleone's push into legitimate businesses.
Sonny (IMHO) would have been just like Mike. In fact the infamous "I don't feel I have to wipe everybody out. Just my enemies" could just have easily been said by Sonny had he become Boss.
Don't get me wrong,I don't think either one of these guys would have been a good pick,but for purposes of discussion, that's my two cents worth.

Something else, too: What kind of enterprise would Vincent have headed? By GFIII, Michael had no connection with the NY "olive oil business" or the casino business.

I think that would have been a great plotline for GF4...how Vincent takes the Olive oil business back...and the drugs, and Immobilaire.

If I'm not mistaken,at the beginning of GF III, Michael is already the single largest shareholder in Immobiliare and controls 6 of the 13 seats on the Board.The purpose of the deal with the Archbishop was to get the Vatican's 25% share,thereby giving Mike majority control.

Since the deal didn't go through,wouldn't Mike still hold his original shares? I don't know how it's done by Italian Corporations,but even if Mike was voted off of the Board,wouldn't they still have to honor his shares,or at least have to buy him out?
I'm not sure what effect the killings at the end had on Immobiliare ,but assuming that it remained a viable Company,I'm thinking that Mike would have still had a nice chunk of change to pass on to the Family.
Any thoughts?